Several problems have been associated to the operation of stators of rotating machines, the effects of which can be detrimental to their yield and operating life. The most important problems have been identified to be: the gathering of deposits that obstruct cooling ducts of stator bars, the loss of insulation between elementary conductors (stator winding) that constitute a stator bar, the loss of quality of the connections of stator bars, and the loss of insulation of stator laminations.
Detection of defects, such as obstructive deposits in a cooling duct of a stator bar, may however be problematic. The liquid circulation cooling system plays a crucial role in the life span of rotating machines. An obstruction, even a very small one, of the cavities of the stator bars causes a considerable elevation of temperature, because the electrical current that circulates therein is very high. A large amount of thermal energy must then be dissipated by the stator bar. The quantity of heat to be dissipated in the stator bar is proportional to the square of the current that circulates therein and is given by the following relationship:PbαI2×R  (1)where    Pb=Heating load to be dissipated by the stator bar (Watts)    I=Current(Amperes)    R=Ohmic Resistance of stator bar (Ohms)
To allow heat to be dissipated, cavities inside of which a liquid circulates are provided in the heart of each stator bar.
Heat transfer between the liquid and a radiator allows heat to be evacuated.
One of the major problems with machines that are cooled by liquid circulation is the accumulation of a deposit on the internal walls of these cavities. This undesirable situation causes a decrease of liquid flow, causing the temperature of the stator bar to increase even more, which can lead to a machine breakdown as a result of a destruction of the insulating material.
Another problem associated with the operation of rotating machines having stator bars is the loss of insulation between conductors (for example: Roëbel bars) of the stator bar.
Each stator bar is made of a plurality of conductors that are insulated from one another (multicore cables) to decrease skin effects. Skin (or pellicular) effects result from an electrical current that does not circulate uniformly in a cross-section of a conductor, but rather on its peripheral portion. Segmentation of a stator bar into a plurality of conductors has the effect of increasing the effective cross-section of the stator bar and, by the same token, decreasing the resistance of the conductor.
Subdividing a stator bar into a plurality of insulated conductors, as seen in FIG. 2, allows for a decrease of skin effects. However this decrease is directly related to the good operating conditions of the insulations. If the insulant becomes deteriorated, there is an increase of skin effects, as well as an increase in electrical resistance and temperature. This loss of insulation takes place mainly at the extremities of stator bars because of the possible movement produced by a leverage effect.
Yet another problem associated to the operation of stator bars is the loss in quality of the connections between stator bars. Stator bars are connected to one another through silver welds. Any deterioration of a connection causes a decrease in machine efficiency. A deteriorated connection point causes an increase of the resistance of the conductor, yielding a temperature increase at the deteriorated site.
Finally, loss of insulation in stator laminations is another problem associated with stator bar operation. The stator nucleus of a machine is made of piles of sheet iron plates that are separated from one another by means of an insulating material. Such an assembly allows for reducing to an acceptable level Foucault currents that are induced by the intense magnetic field generated by the poles of the rotor. If the insulation material is lost in some places, the Foucault currents will intensify in the degraded zone and the local temperature will increase. Such a temperature increase will be harmful to the performance of the machine and may even cause a breakdown following the metal melting.
Whenever one of the above-mentioned problems occurs, the local temperature inside the stator rises at the defective sites. A solution suggested in order to detect the above-mentioned problems has been the mounting, inside the stator, of a large number of temperature probes capable of producing a thermal mapping that covers the whole of the inside of the stator. Such a thermal mapping would allow to determine the probable cause of a defect by detecting an abnormal temperature rise at one or more given points of the thermal cover.
Unfortunately such a theoretical solution is difficult to implement in an actual stator bar environment. It is well-known that strong currents are induced by the elevated currents and the electromagnetic fields that circulate in stator bars. Therefore, the number of temperature probes inside a stator will be limited by the fact that care must taken with respect to the wiring of these probes. Furthermore, the amount of wiring that is required could constitute a safety hazard for the machine operators, as well as hinder the good operation of the machine itself.
While a limited number of such temperature probes may be installed at manufacturing time, the later addition of even a single probe requires major works at a prohibitive cost. For example, such additions would involve the opening of the stator and could, in certain cases, have the added undesirable effect of cancellation of the manufacturer warranty.
There exists therefore a need for a system and method allowing to detect a local elevation of temperature representative of stator defects in a cost-effective manner.
There exists furthermore a need for a system and method allowing early detection of stator defects in a non-intrusive manner so that corrective measures may be administered.